1. How do you think the ocean floor look like?
Have you ever tried to draw something you can’t see? By 1952, geologists Marie Tharp and Bruce Heezen had set to work mapping the ocean floor. Tharp drew details of the ocean floor based on data taken from ships. The data showed how the height of the ocean floor varied. Tharp’s maps, which were first published in 1957, helped to confirm the hypothesis of continental drift.
How do you think the ocean floor look like?

2. Lesson 2: SEA-Floor Spreading
Chapter 5:
Plates Tectonics
Lesson 2: SEA-Floor Spreading

4. What are Mid-Ocean Ridges?
In certain places, the floor of the ocean appears to be stitched together like the seams of a baseball.
Scientists found that the seams formed mountain ranges that ran along the middle of some ocean floors.
Scientists called these mountain ranges mid-ocean ridges.
Mid-ocean ridges form long chains of mountains that rise up from the ocean floor.

5. Sea-Floor Spreading Ocean Floors
Mid-ocean ridges rise from the sea floor like stitches on the seams of a baseball.

6. Sea-Floor Spreading
Mid-Ocean Ridges

7. What is Sea-Floor Spreading?
Mid-ocean ridges continually add new material to the ocean floor in a process called sea-floor spreading.
Sea-floor spreading adds more crust to the ocean floor.
At the same time, older strips of rock move outward from either side of the ridge.

8. Sea-Floor Spreading Sea-Floor Spreading
Some mid-ocean ridges have a valley that runs along their center. Evidence shows that molten material erupts through this valley and then hardens to form the ocean floor.

9. What happens at Deep-Oceans Trenches?
The ocean floor eventually plunges into deep underwater canyons called deep-ocean trenches.
At a deep-ocean trench, the oceanic crust bends downward.
In a process taking tens of millions of years, part of the ocean floor sinks back into the mantle at deep-ocean.

10. What happens at Deep-Oceans Trenches?
Process of Subduction
The process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle again is called subduction.
As subduction occurs, crust closer to a mid-ocean ridge moves away from the ridge and toward a deep-ocean trench.

11. Sea-Floor Spreading Subduction Oceanic crust created
along a mid-ocean ridge is
destroyed at a deep-ocean trench.
During the process of subduction, oceanic crust sinks down beneath the trench into the mantle.

12. What happens at Deep-Oceans Trenches?
Subduction and the Earth’s Oceans
The processes of subduction and sea-floor spreading can change the size and shape of the oceans.
An ocean surrounded by many trenches may shrink.
An ocean that contains more ridges than trenches will probably grow larger.

13. Sea-Floor Spreading Deep-Ocean Trenches The deepest part of
the ocean is along the Mariana Trench. Several trenches in the Pacific Ocean are shown in yellow.